Along with recent advances in vehicle technologies, the variety of rotation angle detectors for detecting a rotation angle of a steering shaft has increased to perform various controls in vehicle operation.
A conventional rotation angle detector is described with reference to FIG. 3. FIG. 3 shows an exploded perspective view of a conventional rotation angle detector. Rotor 1 with spur gear 1A formed outside the periphery has key 1B provided inside the periphery to engage with a steering shaft (not shown) that is inserted through rotor 1.
First detection head 2 has spur gear 2A formed outside the periphery and second detection head 3 has spur gear 3A formed outside the periphery with a different number of teeth than spur gear 2A. Spur gear 2A of first detection head 2 engages with spur gear 1A of rotor 1 and, at the same time, spur gear 3A of second detection head 3 engages with spur gear 2A of first detection head 2.
A plurality of wiring patterns (not shown) are formed on the top and bottom surfaces of circuit board 4 disposed below and approximately parallel to first detection head 2 and second detection head 3. Meanwhile, magnetic sensors 5B and 6B are mounted on the surface of circuit board 4 facing magnets 5A and 6A mounted in the center of first detection head 2 and second detection head 3 respectively.
Magnet 5A and magnetic sensor 5B thus facing each other are to form the first detection section, and similarly magnet 6A and magnetic sensor 6B are to form the second detection section.
Circuit board 7 disposed perpendicularly to circuit board 4 and connected via lead wires or the like (not shown) includes electronic components such as microcomputers, thereby forming controller 8 connected to magnetic sensors 5B and 6B.
Cover 10 made of insulation resin covers the top surface of case 9, also made of insulation resin, so as to cover rotor 1, first detection section 2, second detection section 3 and circuit board 4 or the like; and cover 11, similarly made of insulation resin, covers a side surface of case 9, so as to cover circuit board 7 and controller 8 or the like, thus completing the rotation angle detector.
The rotation angle detector with the above configuration is mounted on a vehicle, controller 8 is connected to an electronic circuit (not shown) of the vehicle via a connector (not shown) or the like, and a steering shaft is inserted through the center of rotor 1. Since the steering shaft engages with key 1B, rotor 1 rotates in response to a rotation of the steering shaft.
In the above configuration, upon rotating the steering shaft when driving a vehicle, rotor 1 starts rotating, then first detection head 2 starts rotating in response to rotation of rotor 1, and second detection head 3 starts rotating in response to rotation of first detection head 2 sequentially. Magnets 5A and 6A mounted in the center of the first and second detection heads 2, 3 rotate as well, and accordingly, magnetic sensors 5B and 6B detect changing magnetic force from magnets 5A and 6A as detection signals in voltage waveform including sine waves and cosine waves.
The detection signals are input into controller 8 in waveform in the form of saw-tooth waves. Since first detection head 2 and second detection head 3 differ in number of teeth or rotation speed, the waveforms from first detection head 2 and second detection head 3 differ in frequency cycles, forming phase-shifted detection signals.
Using these two different detection signals from first detection head 2 and second detection head 3 together with the respective numbers of teeth, controller 8 runs a predetermined calculation to detect a rotation angle of rotor 1 or the steering shaft, which is then output into the electronic circuit of the vehicle for use in various controls in vehicle operation.
Other than the configuration as described above in which spur gears formed outside the peripheries engage with each other, some models have tried to dispose first detection head 2 and second detection head 3 perpendicularly to rotor 1 and to mount magnetic sensors 5B and 6B on a single circuit board 7, by using bevel gears for rotor 1 and first detection head 2 engaged with the rotor 1, thus removing circuit board 4 to realize a low-cost configuration.
In this case, however, first detection head 2 needs to be provided with both a bevel gear to engage with rotor 1 and a spur gear to engage with second detection head 3, which causes not only a complicated componentry but also increase in gear engaging points that tends to cause errors easily owing to the clearance between respective gears or so-called backlash.
Japanese Patent Laid-Open Application No. 2005-156163 discloses a known configuration.
However, there have been problems in the above conventional art in that the rotation angle detector has become expensive owing to the use of two circuit boards 4 and 7, and has had design limitations in dimensions when the diameters of first detection head 2 and second detection head 3 have had to be enlarged to increase the number of teeth so as to improve the detection accuracy of the rotation angle.